The invention relates to optical network communications. More particularly, the invention relates to re-configurable optical add/drop multiplexers.
Recent advances in optical communications technology have provided an optical building block incorporating selectable optical gratings and a circulator. This building block is well suited to building efficient re-configurable optical add/drop multiplexers. FIG. 1 shows such a re-configurable optical add/drop multiplexer (ROADM) building block, generally indicated by 20. The building block provides optical drop capability and comprises an optical circulator 22 connected to a selectable fiber Bragg grating 24. In operation, a wavelength division multiplexed (WDM) optical signal is introduced at input port 26 of the optical circulator 22, which directs the optical signal to a first port 28 of the selectable fiber Bragg grating 24. The selectable fiber Bragg grating 24 is controlled at 30 to reflect a selected wavelength of the optical signal back through the first port 28 to the circulator 22, which directs this reflected wavelength to a xe2x80x9cdropxe2x80x9d port 32 of the circulator 22. The portion of the WDM optical signal, which is not reflected at 30, is passed through the selectable fiber Bragg grating 24 to a second port 34.
FIG. 2 shows a ROADM module, generally indicated by 40. A second circulator 36 is added to the building block of FIG. 1 to provide optical add capability. This configuration allows the selectable fiber Bragg grating 24 to be reused to add an optical signal having the selected wavelength. In operation, the configuration of FIG. 2 behaves similarly to that of FIG. 1. The WDM optical signal minus the dropped portion continues from the second port 34 to the second circulator 36 to output port 38. An optical xe2x80x9caddxe2x80x9d signal having the same wavelength as the selected wavelength is presented at xe2x80x9caddxe2x80x9d port 42 of the second circulator 36 which directs it to the second port 34 of the selectable fiber Bragg grating 24. The xe2x80x9caddxe2x80x9d signal is reflected by the selectable fiber Bragg grating 24 at 30. The xe2x80x9caddxe2x80x9d signal passes back through the second port 34 and through the second circulator 36 to the output port 38. The configuration of FIG. 2 thus provides increased functionality to that of FIG. 1 with only a small incremental increase in cost. It also has the advantage of little additional increase in insertion loss on the through path from input port 26 to output port 38.
Two-fiber optical ring networks typically use fiber pairs to communicate between nodes, one fiber for transmitting and one for receiving. FIG. 3 illustrates two of the ROADM blocks of FIG. 2, shown as 40A and 40B, used to form a bi-directional ROADM node, generally indicated by 43. ROADM block 40A receives WDM signals at 26A from a xe2x80x9cWestxe2x80x9d facing node, drops and adds signals of a desired wavelength at 32A and 42A respectively and sends the WDM signal at 38A to an xe2x80x9cEastxe2x80x9d facing node. Likewise, ROADM block 40B receives WDM signals at 26B from the xe2x80x9cEastxe2x80x9d facing node, drops and adds signals of a desired wavelength at 32B and 42B respectively and sends the WDM signal at 38B to the xe2x80x9cWestxe2x80x9d facing node.
Standard two-fiber (2F) SONET bi-directional line switched rings (BLSRs) require that a failure of node equipment can be handled by normal protection switching. A disadvantage of the configuration of FIG. 3 is that a failure of any of the components of a ROADM block takes the associated fiber path out of service and thus results in a traffic outage. For example, if there is a failure of the fiber Bragg grating 24A or 24B, the optical drop capability ceases to function because the selected wavelength will not be reflected correctly to circulator 22A or 22B. Similarly, the optical add capability stops as well because the optical signal to be added will not be reflected by the fiber Bragg grating 24A or 24B, back to the circulator 36A or 36B and out to the output port 38A or 38B, instead it will continue through the fiber Bragg grating 24A or 24B to the first circulator and will be directed to the xe2x80x9cdropxe2x80x9d port 32A or 32B. Worse still, performing maintenance on the node 43 by replacing components will result in a traffic outage.
FIG. 4 illustrates a bi-directional ROADM configuration having an xe2x80x9cEast/Westxe2x80x9d architectural split. The bi-directional ROADM node is split into a xe2x80x9cWestxe2x80x9d module 44A and an xe2x80x9cEastxe2x80x9d module 44B. Instead of the selectable fiber Bragg grating 24A of module 44A handling wavelength selection for both the xe2x80x9cdropxe2x80x9d port 32A and the xe2x80x9caddxe2x80x9d port 42A, the selectable fiber Bragg grating 24A in xe2x80x9cWestxe2x80x9d module 44A handles only the wavelength selection of the xe2x80x9cdropxe2x80x9d port 32A. A second selectable fiber Bragg grating 24Axe2x80x2 is added in xe2x80x9cEastxe2x80x9d module 44B to select the wavelength to be added at the xe2x80x9caddxe2x80x9d port 42A. A failure in xe2x80x9cWestxe2x80x9d module 44A would appear as a fiber failure, which can easily be handled by the SONET layer through normal protection switching and not affect the entire node. The failure would not affect the function of xe2x80x9caddxe2x80x9d port 42A in xe2x80x9cEastxe2x80x9d module 44B. Similarly, selectable fiber Bragg grating 24B handles wavelength selection for only the xe2x80x9cdropxe2x80x9d port 32B and another selectable fiber Bragg grating 24Bxe2x80x2 handles the wavelength selection for the xe2x80x9caddxe2x80x9d port 42B.
A disadvantage of the configuration of FIG. 4 is that the cost savings advantage of reusing a selectable fiber Bragg grating to provide the wavelength selection for both drop and add functions of a ROADM, as described in relation to FIG. 2 and FIG. 3, is lost.
It is therefore an object of the present invention to overcome the aforementioned disadvantages in the prior art. Accordingly, devices and methods are provided for improved optical add/drop multiplexing.
One broad aspect of the invention provides an optical add/drop multiplexer having an optical wavelength selective device, a first optical circulator and a second optical circulator having a first operating mode and a second operating mode. The optical wavelength selective device has a first port and a second port and is adapted to reflect optical signals having a selected wavelength and to pass optical signals having wavelengths other than said selected wavelength. The first optical circulator has an input port and a drop port and is adapted to direct optical signals from the input port to the first port of the wavelength selective device and to direct optical signals from the first port of the wavelength selective device to the drop port. The second optical circulator has an output port and an add port. The second optical circulator, in the first operating mode, is adapted to direct optical signals from the second port of the wavelength selective device to the output port, and for directing optical signals from the add port to the second port of the wavelength selective device. In the second operating mode, the second optical circulator is adapted to direct optical signals from the add port to the output port.
In some embodiments, the optical wavelength selective device is adapted to select the selected wavelength from a plurality of wavelengths.
In some embodiments, the optical wavelength selective device is a selectable optical grating.
In some embodiments, the optical wavelength selective device is a selectable Bragg grating.
In some embodiments, the second optical circulator is a reversible optical circulator.
Another broad aspect of the invention provides a building block for a bi-directional optical add/drop multiplexer. The building block has an optical wavelength selective device, a first optical circulator and a second optical circulator having two operating modes. The optical wavelength selective device has a first port and a second port and is adapted to reflect optical signals having a selected wavelength and to pass optical signals having wavelengths other than the selected wavelength. The first optical circulator has an input port and a drop port and is adapted to direct optical signals from the input port to the first port of the wavelength selective device and to direct optical signals from the first port of the wavelength selective device to the drop port. The second optical circulator has an output port and an add port. The second optical circulator, in the first operating mode, is adapted to direct optical signals from an external wavelength selective device to the output port, and for directing optical signals from the add port to the external wavelength selective device. In the second operating mode, the second optical circulator is adapted to direct optical signals from the add port to the output port.
In preferred embodiments the building block is integrated on a single substrate.
In some embodiments the external wavelength selective device is a wavelength selective device of a corresponding building block.
Some embodiments of the invention provide an optical network node having at least two of the building blocks.
Some embodiments of the invention provide an optical network having an interconnected plurality of the optical network nodes.
Another broad aspect of the invention provides a method of wavelength management in an optical network. The method involves providing at least one network node with at least a first and a second building block for a bi-directional optical add/drop multiplexer. Each building block has an optical wavelength selective device, a first optical circulator and a second optical circulator having at least two operating modes. The optical wavelength selective device has a first port and a second port and is adapted to reflect optical signals having a selected wavelength and to pass optical signals having wavelengths other than the selected wavelength. The first optical circulator has an input port and a drop port and is adapted to direct optical signals from the input port to the first port of the wavelength selective device and to direct optical signals from the first port of the wavelength selective device to the drop port. The second optical circulator has an output port and an add port. The second optical circulator, in the first operating mode, is adapted to direct optical signals from the second port of the wavelength selective device of the other building block, to the output port, and for directing optical signals from the add port to the second port of the wavelength selective device of the other building block. In the second operating mode, the second optical circulator is adapted to direct optical signals from the add port to the output port.
The method further involves operating each second optical circulator in the first operating mode and when a failure is detected in one of the building blocks, operating the second circulator of the other building block in the second operating mode.